Recovery and purification systems for acrylonitrile and methacrylonitrile obtained by the ammoxidation of propylene or isobutylene are well known. See for example U.S. Pat. Nos. 3,433,822; 3,399,120; 3,535,849; and 3,936,360. The gaseous reactor effluent from an ammoxidation reactor is first directly contacted with a quenching liquid to cool the effluent and remove a substantial amount of contaminates produced during the reaction, such as polymers. The cooled gaseous quench effluent is typically sent to a washing column or absorber wherein the gaseous effluent is contacted with water. The liquid stream from the bottom of the absorber containing the various nitriles, water and some impurities is then sent to a distillation column. Solvent water is used to extractively distill this stream, producing an overhead vapor stream of acrylonitrile. As described in U.S. Pat. No. 3,999,120, the bottoms of the extractive distillation column may then be sent to a second stripping column. The overhead of this stripping column contains acetonitrile with a minor amount of water, and the liquid bottoms stream contains water and impurities. An alternate method of recovery, also found in this reference, is the removal of a sidestream from the extractive distillation column. This stream containing mostly acetonitrile and water, is sent to a smaller stripping column with acetonitrile being removed overhead and the liquid bottoms containing mostly water being returned to the extractive distillation column. When this method of recovery is used the liquid bottoms stream from the extractive distillation column is mostly water and impurities with traces of acetonitrile.
Heat necessary to perform the recovery operations is applied to the bottom of both the extractive distillation column and the stripping column. Unfortunately, this heat, usually applied by using an indirect reboiler, brings about polymerization of the impurities found in the associated columns.
Waste disposal of these bottoms streams is both difficult and costly. The solids content of the stream due to the polymerization is so high that it prevents normal waste water treatment. Further, the size of these streams are very large, thus requiring large specialized equipment. For example, German Pat. No. 050,722 discloses a method for purification of this waste water using a separate distillation column and large amounts of steam to evaporate the water, thus reducing the size of the latter specialized waste treatment facilities.
The present invention provides a unique method for reducing the size of these troublesome streams with very little capital investment or operating costs.